SB 3.11.19
catvari trini dve caikam
krtadisu yatha-kramam
sankhyatani sahasrani
dvi-gunani satani ca
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
The duration of the Satya millennium equals 4,800 years of the years of the demigods; the duration of the Treta millennium equals 3,600 years of the demigods; the duration of the Dvapara millennium equals 2,400 years; and that of the Kali millennium is 1,200 years of the demigods.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
As aforementioned, one year of the demigods is equal to 360 years of the human beings. The duration of the Satya-yuga is therefore 4,800 × 360, or 1,728,000 years. The duration of the Treta-yuga is 3,600 × 360, or 1,296,000 years. The duration of the Dvapara-yuga is 2,400 × 360, or 864,000 years. And the last, the Kali-yuga, is 1,200 × 360, or 432,000 years.